1. Field of the Invention
The present invention is directed to a method for determining the time curve of the basic field of a nuclear magnetic resonance tomography apparatus under switched gradients.
2. Description of the Prior Art
The chronological stability of the basic magnetic field is of decisive importance in nuclear magnetic resonance tomography. Fluctuations during data acquisition lead to artefacts which in a worst case may result in the total unusability of the image. Superconducting magnet systems exhibit a high field stability a priori. Shielding against external disturbances is thereby also good. Pulse sequences employing switched gradients produce eddy currents that also have B.sub.0 field components under certain circumstances, thus altering the basic field. This problem is described, for example, in U.S. Pat. No. 5,289,127. The curve of the B.sub.0 field is thereby measured and stored for particular gradient sequences. In a measurement of image data, either the excitation pulses or the received nuclear resonance signals are phase-modulated in such a way that the time dependence of the basic magnetic field is compensated. This patent also mentions the possibility of carrying out a software correction of the received signals instead of this hardware-oriented correction.
The article by H. Bruder et al., "Image Reconstruction for Echo-Planar Imaging with Non-Equidistant k-Space Sampling," Magnetic Resonance in Medicine, 23, pp. 311-323 (1992), explains a possibility for carrying out a phase correction in the direction of the readout gradient in order to remove influences from basic field inhomogeneities.
The article by T. Onodera et al., "A Method for Measuring Field-Gradient Modulation Shapes. Application to High-Speed Spectroscopic Imaging," SMRM Abstracts, 1986, pp. 1398-1399, proposes a method for measuring the curve of a magnetic field gradient. A gradient is prefixed to the gradient to be measured, in the same direction. The point in time of the maximum of a nuclear magnetic resonance signal is acquired and stored. This measurement is repeated with different amplitudes of the prefixed gradient. The envelope of the echo maxima then represents the time integral of the gradient curve.
A similar method for measurement of the time curve of magnetic field gradients is presented in German OS 37 27 055. The k-space trajectory is thereby formed from the position of the maxima. The chronological gradient curve is obtained by time differentiation of the k-space trajectory.